Circadian Rhythm of Blood Ethanol Clearance Rates in Rats: Response to Reversal of the L/D Regimen and to Continuous Darkness and Continuous Illumination

Phase relationships of the circadian rhythms of blood ethanol clearance (metabolic) rates and body temperature were studied in rats successively exposed to 4 illumination regimens: LD (light from 0800-2000 hr), DL (light from 2000-0800 hr), constant darkness (DD) and, lastly, constant light (LL). After a 4-wk standardization to each regimen, body temperatures were taken at 9 × 4-hr intervals to establish baseline circadian profiles. One week later, groups (N = 8) received 1.5 g/kg ethanol (i.p.) at 6 equally spaced timepoints during a 24-hr span, when temperatures were again measured. Ethanol clearance rates were estimated from decreasing blood ethanol levels sampled every 20 min from 60-200 min after dosing, and the resultant elimination curves were subjected to cosinor analysis. These studies show for the first time that the high amplitude circadian rhythm in ethanol metabolism persists under constant conditions of illumination (DD and LL), demonstrating that it may well be a truly internal circadian rhythm and not a response to exogenous cues of the light/dark cycle. During both LD and DL, maximal and minimal ethanol clearance rates fell near the end of the dark and light phases, respectively, and followed circadian peak and trough control temperatures by approximately 6 hr. A fixed internal phase relationship between the core body temperature and the circadian rhythm in ethanol metabolism is demonstrated, thus establishing the rhythm in body temperature as a suitable and convenient internal marker rhythm for studies of the metabolism of low-to-moderate ethanol doses. These studies demonstrate that the phase relationships of blood ethanol clearance rate and body temperature can be manipulated by the illumination regimen selected, an observation of both basic and practical importance.